There is conventionally known an acceleration sensor that includes, as shown in FIG. 5A, a rectangular parallelepiped weight portion 100 having a movable electrode, a pair of beam portions 101 for rotatably supporting the weight portion 100 substantially at a center in the longitudinal direction of the weight portion 100 and a pair of fixed electrodes 102 and 103 arranged in a spaced-apart opposing relationship with respect to one side and the other side of the surface of the weight portion 100 demarcated by a straight border line interconnecting the beam portions 101 (see, e.g., Patent Document 1). This acceleration sensor detects the acceleration applied to the weight portion 100 by differentially detecting the change in capacitance between the movable electrode (the section of the weight portion 100 facing the fixed electrodes 102 and 103) and the fixed electrodes 102 and 103 caused by the rotation of the weight portion 100 about the border line as a rotation axis. In this acceleration sensor, a recess portion 104 is formed at one side (the right side in FIG. 5A) of the rear surface of the weight portion 100 with respect to the border line so that the weight of the weight portion 100 can become different at one side (the right side) and the other side (the left side) thereof with respect to the border line. Therefore, upon applying acceleration, the moment acting about the border line as a rotation axis is generated in the weight portion 100. In order to prevent the section of the weight portion 100 having the recess portion 104 from being deformed by the ambient stresses, a reinforcing wall 105 for bisecting the recess portion 104 is one-piece formed with the weight portion 100 to extend in the direction parallel to the border line.    [Patent Document 1] Japanese Patent Application Publication No. 2008-544243
The acceleration sensor stated above is capable of detecting acceleration in two directions orthogonal to the rotation axis. The detection sensitivity in the two directions is equalized by setting the angle θ between the perpendicular line extending from the gravity center position of the weight portion 100 to the rotation axis and the surface of the weight portion 100 to become equal to about 45 degrees. In this regard, a method of increasing the area of the movable electrode is adoptable as one means for enhancing the detection sensitivity of the acceleration sensor. If this method is employed, the thickness of the weight portion 100 needs to be increased in order to keep the angle θ at about 45 degrees. This method is not realistic because the increase in the thickness of the weight portion 100 prolongs the duration of an etching step for forming the weight portion 100.
In order to keep the angle θ at about 45 degrees without increasing the thickness of the weight portion 100, it is thinkable to employ a method in which the weight of the weight portion 100 is reduced by cutting away the section of the weight portion 100 existing just below the beam portion 101 as shown in FIG. 55. Use of this method, however, poses a problem in that the weight-reducing thin section of the weight portion 100 is insufficient in strength.